Image-forming apparatuses such as ink-jet printers are widely used. The ink-jet printer ejects an ink (ink droplets) through plural ink ejection orifices (nozzle outlets) provided on a printing head. By a known technique for ejecting an ink through ink nozzles, a thermal energy is applied to an ink in a nozzle in accordance with a driving pulse to cause film boiling of the ink, and the ink is ejected from the nozzle by a bubble formed by the boiling. Many ink droplets are ejected through the nozzle onto a recording medium corresponding to the image to be formed.
For increasing the image recording speed (image-forming speed), some of the ink-jet printers employing the above technique have line heads having multiple ink ejection nozzles respectively and placed perpendicularly to the delivery direction of the recording medium, and the ink is ejected simultaneously the ink ejection orifices (line printer: ref. e.g., Japanese Patent Application Laid-Open No. 2005-238556).
The image-forming apparatuses for forming an image on a recording medium are required to be capable of forming the image in high quality with a high resolution. The aforementioned line printers and the like ink-jet printers can satisfy the requirements. The ink-jet printers do not bring the printing heads into contact with the recording medium in printing to enable stable image recording, advantageously.
Mostly, the above line printer employs a printing head which has ink ejection orifices arranged parallel perpendicularly to the direction of the recording medium delivery. Image formation with plural printing heads, six heads as an example, arranged along the direction of the recording medium delivery is described with reference to FIGS. 15A and 15B and FIGS. 16A and 16B.
FIGS. 15A illustrates schematically six line-printing heads K1, K2, K3, K4, K5, and K6 arranged parallel perpendicularly to the recording medium delivery direction (arrow-A direction). FIG. 15B illustrates schematically a printed image having an undesired black stripe. FIG. 16A illustrates schematically six line-printing heads K1, K2, K3, K4, K5, and K6 arranged parallel perpendicularly to the recording medium delivery direction (arrow-A direction). FIG. 16B illustrates schematically a printed image having an undesired white stripe and an indent which are caused by oblique delivery of the recording medium. Here, the six printing heads are arranged in the order of K1, K2, K3, K4, K5, and K6 from the upstream side of the recording medium delivery, and are assumed to conduct printing in this order. In FIG. 15A and FIG. 16A, the numbered circles denote respectively an ink ejection orifice, the number denoting the arrangement order number of the printing heads. In FIG. 15B and FIG. 16B, the numbered circles denote respectively a picture element formed by the ink droplets ejected from the ejection orifices of the printing heads of that order numbers. The two-dot chain lines on the recording medium P denotes raster line zones mentioned later.
After one cycle of printing with printing heads K1-K6 in this order, the next printing cycle is repeated successively. During the time between the printing by K6 and the next printing by K1, the recording medium is delivered by the distance corresponding to the positional interval between the printing heads K1 and K6. Since the timing of the printing by K1-K6 (ink ejection timing) can be adjusted by confirming the printed image on the recording medium, various methods are disclosed for correcting the error caused by the printing head.
In the printing as described above, further increase of the image-forming speed can cause positional deviation between the print zone with the printing head K1 and the print zone with the printing head K6 to lower the image quality owing to decline of accuracy in the delivery of the recording medium. That is, further increase of the speed of delivery of the recording medium to increase further the image formation speed will make significant the positional deviation of the picture elements in the delivery direction. The ink droplets ejected from printing head K1 can partly overlap with the ink droplets ejected from the printing head K6 to give rise to a black stripe as shown in FIG. 15B by the following causes: (a) variation in the speed of the delivery belt owing to decentering of the driving roller in the perimeter direction; (b) variation in the speed of delivery belt owing to slippage between the driving roller and the delivery belt; (c) variation in the sheet-delivery speed owing to floating of the recording medium; and (d) variation in the sheet-delivery speed owing to slippage between the recording medium and the delivery belt.
Further, the recording medium P can be delivered obliquely (in the direction shown by two-dot chain line arrow in FIG. 16B: the intended delivery direction being shown by the full line arrow A) to cause deviation in the delivery direction from the intended direction by the following causes: (e) difference in the delivery speed of the recording medium between the both side ends in the breadth direction (in particular when the delivery speed is not constant), and (f) snaky movement of the delivery belt; and so forth. The deviation induced by the above causes prevents precise printing at the intended print position. When the position of the recording medium deviates abruptly, an indent L of the print perpendicular to the delivery direction, or a white stripe will be caused abruptly between a zone printed by the printing head K1 and an adjacent zone printed by the printing head K6 as illustrated in FIG. 16B to lower the image quality.